Fluid pumps, whether for liquids or gases, may be of the axial type, wherein a plurality of cylinders and pumping pistons are aligned parallel to and disposed around a central axis. The pumping pistons are actuated successively and with their strokes overlapping in time to provide continuous pumping of the working fluid.
One method and means of controlling piston actuation is to provide a wobble plate or swash plate, which is tilted relative to the pump axis. The plate engages the pumping piston and cylinder assemblies so as to actuate each one successively as rotation, of the wobble plate on one hand or the piston and cylinder assembly on the other hand, takes place.
Typical adjustable wobble plate designs for axial pumps generally make use of a tilt platform with a pin-ended bearing support along the tilt axis. An external mechanism is then used to rotate the pin-ended platform. This configuration requires the tilt platform and pin-ended bearing structures to support the full pump thrust loads. Structural rigidity and dynamic performance are compromised with an accompanying increase in pump vibration, noise, and small stroke dynamic stability. An unnecessarily large pump envelope is required to accommodate this approach adding to pump cost and size while further exacerbating rigidity and noise problems.
If the mechanism is not sufficiently rigid, pumping load distortions of the cantilevered swash plate can be significant and of the same order as the stroke for small displacement. This leads to control instability. If the mechanism is sufficiently rigid it is also likely to be heavy and reduce response time as well as increase weight and cost.
A further difficulty with present axial configurations is that the tilt mechanism pivot axis used for the swash plate does not pass through the center of the plane of the swash plate. The yoke is then controlled by a hydraulic tilt cylinder or other thruster at one edge. This provides for the most convenient configuration under the circumstances but results in increasing offset of the swash plate centerline as the swash plate is tilted. Special bearing and support mechanisms must then be added to compensate for this and to force the centered alignment of the piston rods. In some cases centering is maintained by a ball and socket arrangement with a ball attached to the rotating drive shaft and a mating socket on the swash plate. This approach, which is common, introduces undesirable axial loading as well as structural bending moments to the shaft.
Further, the offset swash plate pivot swings the swash plate from side to side requiring additional side clearance within the case for the swinging yoke basket, which adds further to weight and cost.
By replacing the suspended pin-ended yoke assembly with a double wedge mechanism, as provided in U.S. Pat. No. 5,724,879, a more robust, compact and rigid tilt platform is presented to the spinning swash plate. One embodiment of this approach is shown in FIG. 1. This provides several advantages over conventional approaches:
Smaller, simpler, more robust case and pump assembly
Potentially lower cost
Clearance volume need not change with tilt
No external actuators required to achieve tilt
Hydraulic support of the swash plate is possible
Potential for reduced noise
Center of the swash plate remains fixed at all tilt angles
Large tilt angles (stroke) are easily achieved
Control forces need only rotate the wedges
No pin-ended yoke bearings
No high point-load bearing problems
No assembly alignment problems
The double wedge assembly is very adaptable to axial pump design. It can provide the basis for a totally new pump design, or be easily adapted to retrofit many existing axial pumps.
FIG. 1 shows a sectional view of a typical retrofit installation using a stroke control assembly and a double wedge assembly. To retain the use of all other pump components without modifications, the wedge stack must be thick enough to present the tilt plane at the same center location as the original pump yoke assembly. A pump retrofit design using the compact wedge assembly may have a shorter case as well as a smaller diameter case while still retaining the original pump assembly of ports, cylinders and pistons.
Generally, the system provides a compact, rigid and robust replacement for typical pin-ended yoke tilt assemblies for adjustable axial pumps. This improved rigidity and stiffness should reduce vibration and noise. The inherent simplicity should also lead to lower cost of production while improving durability. In design of an actual system, specific analysis of specific frictional moments, static and dynamic loading and hydraulic control parameters will be required.
In addition, typical axial devices have no means for controlling clearance volume of the device. In typical axial pumps the clearance volume increases with decrease in stroke, a situation particularly not desirable in pumping gaseous fluids. Control of clearance volume would have utility in axial devices that use a gas as the working fluid. Such control would be of particular value in air-conditioning and refrigeration systems.
It is one objective of this invention to provide a tiltable flat surface platform wherein pump thrust loads are directly and hydrostatically supported, thereby eliminating the need for pin-ended bearing support along or perpendicular to the tilt axis and the other pump design considerations required of pin-ended bearing support.
It is another objective of this invention to provide a hydrostatically supported tiltable flat surface platform wherein the zero tilt position is moveable with respect to the pumping piston and cylinder assembly thereby providing a means for controlling clearance volume, as well as displacement, of the device. This feature will be of particular utility in gas compressors used in air-conditioning and refrigeration systems and more specifically in such systems where a transcritical gas such as carbon dioxide, i.e., CO2, is used as the refrigerant.
A new method and apparatus for control of stroke and clearance volume in axial devices is presented which corrects typical shortcomings while offering new possibilities to axial device performance. The new control module is compact, self-contained, and without the need for external tilt control mechanisms. By providing a new adjustable wobble plate with hydraulic support, pin-ended bearings and cantilevered tilt platforms are no longer needed. Device rigidity is improved and vibrations are absorbed by the hydraulic support and device envelope size and noise are minimized. Additional possibilities are then available to dynamically control device timing, promising further improvements in device performance and noise control. These and other advantages of the invention will be apparent from the following detailed description with reference to the appended drawings.